FIG. 1 illustrates an aircraft engine 3 of the unducted fan type, in which the invention can be used. Region 5 is shown in FIG. 2, wherein counterrotating turbines 9 (hatched) and 12 (plain) are driven by a hot gas stream 14 provided by a core engine (not shown). The turbines 9 and 12, in turn, drive counterrotating fan blades 15 and 18 shown in FIGS. 1 and 2. (The term "counterrotating" means that turbines 9 and 12, as well as blades 15 and 18 to which they are attached, rotate in opposite directions, as shown by arrows 25 and 27 in FIG. 1.)
A view of sub-region 6B in FIG. 2 is shown in FIG. 3, and in more detailed form in FIG. 4. The turbine blades which are located, but not shown, in sub-region 6B in FIG. 2 are shown schematically in FIG. 3 as blades 28 and in detail in FIG. 4. The blades 28 extend between a casing 24 and an inner barrel 41 in FIGS. 3 and 4. The blades extract energy from the gas stream 14 in FIG. 2, and also act as a stiff connecting web between the casing 24 and the barrel 41.
The fan blades 15 in FIG. 3 are supported by a structure which is shown as a ring 22 in FIG. 4, and which is fastened to the casing 24 by brackets 36. During operation, the centrifugal load of the fan blades 15 is carried by the ring 22 as a hoop stress. The brackets 36 transmit the thrust load generated by the fan blades 15 to the casing 24. Other structures, not concerned with the present invention, carry the thrust load to the aircraft in FIG. 1.
One type of bracket is described in the U.S. patent application entitled "Flexible Connector for Use in Aircraft," by Wakeman et al., which was filed on Dec. 19, 1988, having Ser. No. 286,101. This application is hereby incorporated by reference.
The actual ring structure used is not the circular ring 22 shown in FIG. 4, but is a polygonal ring 22P shown in FIG. 5. The polygonal ring 22P includes two types of sections: one type is a blade support section 22B, or "hub," also shown in FIG. 5A, which includes thrust bearings 22D which transmit the centrifugal load imposed by the fan blade 15 to the ring 22P. The thrust bearings 22D allow pitch change of the blade to occur, as indicated by arrow 23.
The other type of ring section is a connector section 22A which connects neighboring hubs 22B. The connector includes rails 29 which are in tension because of the centrifugal load of the blades 15.
One type of polygonal ring is described in the U.S. patent application entitled "Blade Carrying Means", filed by Hauser, Strock, Morris and Wakeman on Nov. 2, 1984, and having Ser. No. 667,663 now U.S. Pat. No. 4,863,352. This application is hereby incorporated by reference.
The polygonal ring also carries a cowling 40 in FIGS. 1 and 2. One type of rotating cowling is described in the U.S. patent application entitled "Rotating Cowling," filed by Hermans and Wakeman on Apr. 11, 1989, having Ser. No. 07/336,375. This application is hereby incorporated by reference.
One arrangement of the turbine/polygonal ring/fan blade/cowling system is shown in FIG. 6. Several features of this system will now be discussed. One, the temperature of the turbine casing 24 can range between ambient temperature when the engine is not running, and a temperature of 1,000 degrees F. during operation. The latter, higher temperature, results from contact with the hot gas stream 14 in FIG. 2 passing through the turbine. This wide swing in temperature causes the circumference of the casing to change in dimension. This dimension change causes the legs 42 in FIG. 5 of the brackets 36 to separate: the legs move to phantom positions 42P: distance 47 has increased to distance 47P. This change in distance causes strain in the bracket 36 at the base region 48.
A second feature of the system is that the brackets extend axially beyond the rails 29 of the polygonal ring, as indicated by distance 50 in FIGS. 5 and 6. This extension beyond rails 29 causes the forging envelope of the polygonal ring to be large. The forging envelope 22E in FIG. 14 (envelope is shown hatched) for the polygonal ring 22P has an axial length L. The forging envelope refers to the rough casting 55 in FIG. 8 which is the precursor of the final forged bracket 36. The envelope 55 is generally larger than the final bracket 36, because the envelope 55 is shaped and trimmed during the forging process.
It is desirable to reduce the size of both forging envelopes.
A third feature of the system is that the cowling 40 is fastened to the ring by other, cowling brackets 57 shown in FIG. 6. The centrifugal load of the fan blades 15 in FIG. 4 stretches the polygonal ring 22P in FIG. 5, thus increasing the diameter of the ring. This diameter increase places added stress upon the cowling, through the cowling brackets 57, because the cowling is subject to no similar diameter increase because of centrifugal load. An example will illustrate the magnitude of the centrifugal forces involved.
It is assumed that the propeller diameter, dimension 60 in FIG. 3, is 12 feet. It is assumed that each fan blade can be treated as a point mass 64 weighing 54 pounds and located on the circumference of a circle 67 which is six feet in diameter. It is also assumed that the speed of rotation is 1200 rpm, or 20 revolutions per second, which corresponds to (2)(pi)(20) radians per second, i.e., about 126 radians per second.
Centrifugal acceleration is equal to w.sup.2 r, wherein w is angular velocity (radians per second) and r is radius. In this example, the acceleration is about 47,374 feet per second.sup.2 : EQU 47,374=[126/sec].sup.2 .times.3 feet
Dividing this number by the acceleration due to gravity, 32.2 feet per second.sup.2, gives a quotient of about 1471. The quotient is the g field experienced by the point masses.
Stated another way, each point mass 64 (representing the weight of each blade), which originally weighed 54 pounds, now weighs about 80,000 pounds under centrifugal force (1471.times.54=79,434). This centrifugal force causes the ring diameter to increase, as well as to assume a slightly polygonal shape, with the blades 15 located at the polygon's vertices. The diameter increase tends to bulge the cowling outward.
A fourth feature of the system concerns a result caused by the type of mounting by which the ring 22P in FIG. 5 is fastened to the casing 24 in FIG. 4. The particular view of the turbine stage shown in FIG. 4 indicates that all turbine blades are identical in shape. However, such is not the case. The blades, such as blades 28A in FIG. 4, which are directly radially inward of the propeller blades, are larger in cross section, as indicated by blade 28A in FIG. 4A, than are the other blades 28. The blades 28A of larger cross-section carry much of the load which the propeller blades 15 apply to the turbine, and, in so doing, act as struts.
It has been found that high oscillatory stresses can exist in these struts during operation. It is possible that these stresses may have been caused by the offset 71 in FIG. 6 between the mounting point 73 of the bracket 36 and the axis 74 of the strut.